The present invention relates to a direct injection internal combustion engine of a compression ignition type in which fuel is injected into a cavity formed in a piston to form an excellent mixture gas.
A high-speed direct injection diesel engine of relatively small size is known which has a toroidal combusion chamber in the form of a semi-cylindrical cavity in the top of a piston. A conical protrusion is formed at the center of the bottom of the cavity to provide an annular recess. A multi-hole nozzle (having four or five holes) is arranged, as a fuel injection valve, at the center of the cavity. The nozzle injects a spray of fuel radially, and the fuel evaporates while passing across a swirl stream formed in the cavity, thus forming mixture gas.
The fuel spray injected by the nozzle has a large velocity and a strong penetration force in the axial direction of the nozzle hole. Therefore, while passing through the high temperature air in the cavity, the fuel spray evaporates gradually, the diameters of the fuel droplets in the spray decrease, and the flight velocity of the droplets also decreases. Accordingly, the fuel spray is caused to flow by the swirl stream formed in the cavity, thus forming mixture gas in the swirl stream near the downstream walls of the cavity.
Immediately after this conventional diesel engine is started, namely when the engine is operating at low temperature, the temperature of the walls of the cavity is also low, the air temperature is not elevated during the compression stroke, and thus the temperature of the swirl stream formed in the cavity is not so high. Accordingly, the speed of evaporation of the sprayed fuel droplets is low. In other words, the fuel spray does not sufficiently evaporate, as a result of which large unevaporated fuel droplets strike against the walls of the cavity. Accordingly, smoke is generated, the rate of pressure rise is high, the noise level is high, and undesired hydrocarbons (HC) are formed. These are disadvantages accompanying the conventional diesel engine.
Also, for normal low-speed operation, the swirl stream formed in the cavity is weak, and in this case too the sprayed fuel droplets strike against the walls of the cavity causing the same difficulties as for cold running.
The swirl stream in the cavity is generally formed by an intake port which is helical in configuration, namely, a helical port. However, during low speed operation of the engine in which the piston is moving at a low speed, the velocity of the swirl stream is low, and therefore the swirl stream cannot sufficiently decelerate the radial movement of the fuel spray imparted by the multi-hole nozzle so that in this case as well the sprayed fuel droplets strike against the walls of the cavity.
On the other hand, during high speed operation of the engine, the piston is moving at a high speed, and therefore the velocity of the swirl stream formed in the cavity is high. In this case, mixture gas layers formed near the walls of the cavity are caused to flow; that is, the mixture gas layers formed by adjacent injection holes tend to overlap, forming a mixture gas of excessively high concentration, with the result that smoke is generated.
In small engines for automobiles, and especially in engines whose cylinder diameters are of the order of 75 to 100 mm, the range of engine running speed is wide; the idling speed is 500 to 800 rpm and the maximum speed is 4000 to 5000 rpm. Thus, such engines have a problem of forming a mixture gas of excessively high concentration.
The fuel spray injected by only one of the injection holes of the multi-hole nozzle will now be considered. The sprayed fuel droplets have a generally very high velocity, which causes air to be pushed in front of the spray. The velocity of the fuel spray decreases as the fuel spray evaporates. As a result, fuel spray is caught up in and driven by the swirl stream, thus forming mixture gas. Ignition is initiated near the high temperature wall of the cavity near the side surface of the front end of the spray. There is a sufficient amount of air on the surface of the fuel spray, which is in the form of a solid cone, and therefore after ignition near the front end of the fuel spray, not only the mixture gas layer located downstream of the swirl stream but also the surface layer of the fuel spray which has been just injected is combusted. That is, immediately after ignition the velocity of combustion is so high that there is no time to efficiently use the available intake air in the cavity, and accordingly smoke is liable to be generated. Furthermore, as the rate of pressure rise is high, the noise of combustion is also large.
In another type of conventional direct injection diesel engine, a combustion chamber is provided by forming a three-quarter spherical cavity in the top of the piston. The air is swirled by a helical port so that a swirl stream is formed in the cavity at the end of the compression stroke. This diesel engine has found practical use in some fields.
In this diesel engine, the fuel, injected by a single-hole nozzle or a double-hole nozzle, is sprayed against the walls of the cavity, thus forming a fuel film on the wall. The fuel film is evaporated on the cavity wall. The vapor of the fuel thus evaporated is driven by the swirl stream inside the cavity to form a mixture gas.
Compared with the first-described diesel engine having a toroidal combustion chamber, the second-described diesel engine is advantageous in that, as only the fuel vapor which is formed by evaporation of the fuel film is mixed with air for combustion, the quantity of smoke generated is small. However, the latter diesel engine is still disadvantageous in that the combustion characteristics are strongly affected by the temperature of the wall, the engine is difficult to start at low temperatures, and a large amount of noxious emissions, particularly hydrocarbons, is exhausted immediately after starting.
In order to solve the above-described problems accompanying conventional small compression ignition type direct injection internal combustion engines, the inventors have conducted systematic experiments, analyses and trial manufacture, and accomplished this invention.